Environment preservation has become more and more important through the entire world. As been discovered, NO.sub.x is the major cause of acid rain. In fact, almost all NO.sub.x comes from burning fossil fuels. As a result, stringent regulations to reduce the allowable emissions of nitrogen oxides are being promulgated in many industrial areas of the world. Examples are listed in table I.
TABLE I ______________________________________ effective as from 1993 NO.sub.x emissions standards for different kind of fuels in several countries (unit: ppm) coal oil gas dry, O.sub.2 % ______________________________________ R.O.C. 500 400 300 6 *(350) *(250) *(150) Japan 250 150 100 6 U.S.A. 382 236 78 3 Germany 213 106 106 3 ______________________________________
The combustion industry is faced with the necessity of having to reduce nitrogen oxides from its existing units. Under such stringent regulations, conventional combustion technologies are not capable of meeting standards for low NO.sub.x emissions. For this reason, methods for reducing nitrogen oxides in furnaces have been developed. These methods can be divided into two groups: combustion modification and post-treatment. Combustion modification means reducing the NO.sub.x contained in flue gas by way of low NO.sub.x combustion technologies, for instance, the present invention. On the other hand, post-treatment methods treat the flue gas by adding reducing agents, like ammonia or urea, for reducing the nitrogen oxides to nitrogen. Examples include processes of selective catalyst reduction and selective non-catalyst reduction.
The formation of NO.sub.x in the combustion process consists of thermo-NO.sub.x and fuel-NO.sub.x. Thermo-NO.sub.x mostly depends on the peak temperature of the flame. Fuel-NO.sub.x is decided by the nitrogen content of the fuel and the mechanism of the combustion reaction. Nowadays, methods for reducing NO.sub.x emissions by the combustion modification include:
1. changing the operating conditions of the combustion system by:
(a) decreasing the amount of excess air. More excess air means higher oxygen density during combustion, which is beneficial to the formation of NO.sub.x. Therefore, by decreasing the amount of excess air to operate the combustion system nearly under the condition of complete combustion is helpful to reduce the NO.sub.x emissions. In addition, due to the reduction of the amount of air, less heat is taken away by the flue gas, resulting in an increased combustion efficiency.
(b) lowering the heat load or increasing the space for combustion. This leads to an increased heat transfer rate and a lower combustion temperature, so as to reduce the formation of thermo-NO.sub.x. The shortcomings are the diminished capacity of the furnace and poorer economic efficiency.
(c) lowering the pre-heat temperature of the air. This effectively lowers the flame temperature and thus reduces the thermo-NO.sub.x. From the point of view of energy saving, this will cause the loss of useful energy.
2. modifications to the burner or the combustion system, comprising:
(a) staging air combustion. Air is injected into the combustion system at different positions. The central region of the flame forms a fuel-rich reduction area, which inhibits the formation of NO.sub.x. This can slow down the mixing rate of the air and the fuel, which lowers the peak temperature of flame, and then reduces the NO.sub.x.
(b) swirl combustion. Air is guided into the furnace by a swirler. The swirling air flow delays the mixing of the air and the fuel, and forms a recirculation area at the central region, thus lowering the peak temperature of the flame, and reducing the NO.sub.x.
(c) reburning. The combustion process is divided into a main combustion area, a reburning area, and a burnout area. The main combustion area is supplied with 80% of the fuel and kept under a fuel-lean condition. In the reburning area, 10% to 20% of the fuel is injected downstream from the main combustion area, to create a fuel-rich reduction area. After that, in the burnout area, 0 to 10% of the fuel and abundant air are supplied to burn out all fuel particles that have not burned in the previous areas.
(d) flue gas recirculation. A part of the exhaust gas is cooled and guided back to mix with fresh air and then sent into the burner. The flame temperature can be lowered, the oxygen is diluted, and the NO.sub.x is reduced.
Generally speaking, the design principle of a low NO.sub.x burner can be one or a combination of the methods and techniques mentioned above. Such a burner should be operated under a low excess air condition. Regarding the gas-fueled burner, the major source of NO.sub.x is the thermal-NO.sub.x, therefore the reduction of thermal-NO.sub.x is to be taken as the first goal. For the oil-fueled burner, due to the nitrogen contained in the fuel, the reduction of fuel-NO.sub.x should be considered simultaneously. Nevertheless, the mechanism of formation of fuel-NO.sub.x is more complex than that of thermal-NO.sub.x. There are no well developed technologies capable of eliminating fuel-NO.sub.x completely, so the NO.sub.x emissions of the oil-fueled burner are still higher than those of the gas-fueled burner.